The advantages of housing cables and pipes in ducts for the various purposes outlined above have long been recognized and various forms of such ducts have been proposed. Some of the advantages and prior art are more fully set out in my published international patent applications numbers PCT/ZA01/00204 and PCT/ZA2004/000019 published under international publication numbers WO 02/054553 and WO 2005/045294, and the content of which is included herein by reference.
Broadly speaking, the prior art proposals describe a duct that is formed as two opposed semicircular cross-sectioned channels the edges of which are engaged with each other to form a duct to enclose cables or pipes therein. The semicircular cross-sectioned channel shaped duct unit may have co-operant tongue and groove formations extending along the two free edges such that two identical units can have their co-operant edges inter-engaged with each other with the tongues of the formations along one edge fitting, and, in some cases, clipping, into the grooves of the formations along the other edge. The channels are held in their operative relationship with each other by means of clamps or straps that encircle the resultant duct. Alternatively, or in addition, the clipping action of the tongues into the grooves may serve to locate the two channel shaped duct units together. Such a clipping action may make it difficult to separate the two halves for maintenance or subsequent introduction of additional cables or service conduits.
The channel shaped duct unit, in some instances, is extruded, in which case the outer surface is typically smooth to cooperate with straps encircling the closed duct. These arrangements do not generally, however, provide any particular way in which the adjacent ends of two collinear units can be easily attached. They also do not provide any means on the outside of the extrusions to indicate desirable or recommended spacing of encircling straps and where those straps should be located.
In other instances, the duct units are made as injection moldings of predetermined lengths. In this instance external circumferentially extending locating grooves or recesses are provided to indicate fixed spaced positions along the length of the units at which straps or clamps for holding the two duct units together are to be located. However, with fixed length units and fixed locations for straps or clamps, fitting an integral number of units into a particular distance to be covered becomes a problem, as does the in-line connection of cut units in end to end relationship with an uncut length. In addition, if different lengths of units are to be manufactured, the capital cost of multiple injection molding dies becomes exorbitant.